Feline leukemia virus (FeLV) - induced lymphosarcoma in cats is a unique model to study the immune mechanisms involved in the initiation and progression or diminution of neoplastic cell growth. We have now shown that FeLV-infected cats with lymphosarcoma are hypocomplementemic and their sera contain high levels of circulating immune complexes (ClCs). Our studies indicate that the complexes contain FeLV and immunoglobulin IgG. Both FeLV and FeLV containing complexes were shown to activate feline complement in vitro. Further, feline LSA was successfully treated by plasma therapy (the infusion of normal, unheated feline plasma) implicating feline complement as an active factor in tumor cell regression. More recently using an extracorporeal immunosorption procedure using Staphylococcus aureaus, rapid regression of tumor and a general clinical improvement in treated animals was observed. We propose to further our studies on the role of ClCs and complement in feline LSA. Sera from large numbers of normal nonviremic, normal viremic, LSA nonviremic and LSA viremic cats will be screened for ClCs, complement components and immune complex deposition in the kidneys. Finally, our studies on the destruction of leukemic cells following plasma infusion and immunosorption procedure using Staphylococcus aureus will be continued with the aim of ascertaining the molecular mechanisms involved in tumor cell destruction. Results of these studies will lead to a greater understanding of the interplay between the immune system and neoplasia and are thus directly applicable to human cancer.